PROJECT SUMMARY Among the top priorities of the HIV field is the search for therapeutic interventions that can lead to sustained antiretroviral therapy (ART)-free HIV remission. Although the majority of HIV-infected persons will experience rapid viral rebound after ART interruption, there are rare individuals, termed post-treatment controllers (PTCs), who demonstrate sustained virologic suppression for months or years after treatment cessation. These individuals are considered an ideal example of durable HIV control, with direct implications for HIV cure research. However, our understanding of the virologic determinants of HIV remission remains incomplete. This is in part due to the scarcity of PTCs identified through any one research center or clinical trial, and in part because of the limited scope of viral reservoir studies that have been performed to date. The main goal of this proposal is to perform an in-depth virologic analysis of PTCs and our overarching hypotheses are that HIV reservoir characteristics can provide mechanistic insights behind their ability to achieve HIV remission. Through an international effort, we have established the Control of HIV after Antiretroviral Medication Pause (CHAMP) study, the largest study of PTCs world-wide. Our initial studies of PTCs from the CHAMP cohort has already yielded intriguing findings, including the identification of the first HIV reservoir biomarker, total proviral genome numbers, that predicts which individuals will become PTCs after treatment interruption. While these results are promising, the HIV reservoir is far more complex than just the total number of proviral genomes in the peripheral blood mononuclear cells (PBMCs). The activity and impact of the HIV reservoir must take into account a constellation of factors in a conceptual framework that we call the ?HIV reservoir ecology?. This paradigm incorporates the combined proviral, cellular and molecular circuits that contribute to HIV persistence, including: 1) diversity of proviral quasispecies; 2) distribution within cellular subsets and tissue compartments; 3) transcriptional regulation; and 4) the chromosomal environment of the HIV integration sites in the host genome. Using novel assays developed in the laboratories of the proposing investigators, we propose to take a holistic approach in characterizing the HIV reservoir ecology in both PTCs and non-controllers (NCs). The results of such studies will advance our understanding of the mechanisms of HIV control in PTCs, with implications for predicting post-treatment control and determining which of these circuits may be the best targets in the design of HIV remission strategies for all individuals living with HIV.